1. Field of the Invention
The present invention relates to control systems for induction motors.
2. Description of Related Art
In recent years, there has been substantial research on advanced control systems for controlling induction motors without rotational transducers to achieve high dynamic performance of induction motor drives. A major element of these control systems has been the estimation of the rotor position or the position of the rotor flux without using a rotor position sensor. The techniques used for these purposes can be divided into two groups: a) tracking machine spatial saliencies and b) using a variety of control methods for speed and flux estimation.
The spatial saliencies tracking methods require injection of the separate high frequency carrier signal and work best at low speed. They are based on the accurate analysis of machine physical structure irregularities and have intrinsic limitations originating from the requirements of separation of fundamental and carrier frequencies and detection of low amplitude spatial information signal.
The control methods from the second group rely on different control techniques. One of the approaches is based on augmentation of the plant to include dynamics of unknown parameters and use of state dependent Riccati equation filter methodology. A particular control method employs two complementary speed and flux observers or estimators. The main high gain speed and flux estimator works in the motoring and generating region and requires the knowledge of motor speed sign. The auxiliary estimator operates in generating and braking modes but can be justified if motor speed varies sufficiently slowly. However, the switching between the two estimators can cause divergence when the induction motor is operated at low speed under large load. The high gain estimator is utilized in a reference frame related with a flux estimation vector. Rigorous closed loop analysis taking into account rotor resistance variation, and error in estimation of rotor flux and speed provides conditions under which asymptotic torque tracking is achieved.
An object of the present invention is to provide method and apparatus for controlling an induction motor using pseudo (filtered) electrical current and voltage signals and observable time derivatives thereof as inputs to a system of linear algebraic equations representing motor dynamics.